Polyester production system employing hot paste to esterification zone

ABSTRACT

A polyester production process employing an esterification system that uses a distillation column to recover alcohol produced from an esterification zone and then utilizes the recovered alcohol to form a paste, which is recirculated back to the esterification zone with little or no cooling.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system for producing melt-phase polyesters.In another aspect, the invention concerns an esterification systememploying a reactant-containing paste requiring little or no cooling.

2. Description of the Prior Art

Melt-phase polymerization can be used to produce a variety ofpolyesters, such as, for example, polyethylene terephthalate (PET). PETis widely used in beverage, food, and other containers, as well as insynthetic fibers and resins. Advances in process technology coupled withincreased demand have lead to an increasingly competitive market for theproduction and sale of PET. Therefore, a low-cost, high-efficiencyprocess for producing PET is desirable.

Generally, melt-phase polyester production facilities, including thoseused to make PET, employ an esterification stage and a polycondensationstage. In the esterification stage, polymer raw materials (i.e.,reactants) are converted to polyester monomers and/or oligomers. In thepolycondensation stage, polyester monomers exiting the esterificationstage are converted into a polymer product having the desired finalchain length.

In most conventional melt-phase polyester production facilities,esterification is carried out in one or more mechanically agitatedreactors, such as, for example, continuous stirred tank reactors(CSTRs). However, CSTRs and other mechanically agitated reactors have anumber of drawbacks that can result in increased capital, operating,and/or maintenance costs for the overall polyester production facility.For example, the mechanical agitators and various control equipmenttypically associated with CSTRs are complex, expensive, and can requireextensive maintenance. Further, conventional CSTRs frequently employinternal heat exchange tubes that occupy a portion of the reactor'sinternal volume. In order to compensate for the loss in effectivereactor volume, CSTRs with internal heat exchange tubes require a largeroverall volume, which increases capital costs. Further, internal heatexchange coils typically associated with CSTRs can undesirably interferewith the flow patterns of the reaction medium within the vessel, therebyresulting in a loss of conversion. To increase product conversion, manyconventional polyester production facilities have employed multipleCSTRs operating in series, which further increases both capital andoperating costs.

Thus, a need exists for a high efficiency polyester process thatminimizes capital, operational, and maintenance costs while maximizingproduct conversion.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, there is provided a processcomprising: (a) producing an alcohol-containing liquid from adistillation zone; (b) mixing at least a portion of thealcohol-containing liquid with solid particles of an acid to therebyprovide a paste; (c) combining the paste with a recirculation liquid tothereby provide an esterification feed; and (d) subjecting at least aportion of the esterification feed to esterification in anesterification zone, wherein the temperature of the paste when combinedwith the recirculation liquid is not more than 150° C. cooler than thetemperature of the alcohol-containing liquid when exiting thedistillation zone.

In another embodiment of the present invention, there is provided aprocess comprising: (a) subjecting an esterification feed comprisingterephthalic acid and ethylene glycol to esterification in a firstesterification zone defined within the tubes of a shell-and-tube heatexchanger to thereby produce a first product having a conversion of atleast about 50 percent; (b) subjecting at least a portion of the firstproduct to vapor/liquid separation in a disengagement zone definedwithin a disengagement vessel to thereby produce a second liquid productand a second vapor product, wherein the second liquid product has ahigher conversion than the first product; (c) subjecting at least aportion of the second vapor product to distillative separation in adistillation zone to thereby produce a third vapor product and a thirdliquid product, wherein the third vapor product comprises at least about50 mole percent water and the third liquid product comprises at leastabout 50 mole percent ethylene glycol; (d) mixing at least a portion ofthe third liquid product with solid particles of terephthalic acid tothereby provide a paste; and (e) combining at least a portion of thepaste with a recirculation liquid to thereby provide a combined stream,wherein the temperature of the paste when combined with therecirculation liquid is not more than 150° C. cooler than thetemperature of the third liquid when exiting the distillation zone,wherein the recirculation liquid comprises at least a portion of thesecond liquid product, wherein the esterification feed comprises atleast a portion of the combined stream.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present invention are described in detailbelow with reference to the enclosed figure, wherein:

FIG. 1 is a schematic depiction of an esterification system configuredin accordance with one embodiment of the present invention and suitablefor use in a melt-phase polyester production facility.

DETAILED DESCRIPTION

The present invention can be employed in melt-phase polyester productionfacilities capable of producing a variety of polyesters from a varietyof starting materials. As used herein, the term “polyester” alsoincludes polyester derivatives, such as, for example, polyetheresters,polyester amides, and polyetherester amides. Examples of melt-phasepolyesters that can be produced in accordance with the present inventioninclude, but are not limited to, homopolymers and copolymers ofpolyethylene terephthalate (PET), PETG (PET modified with1,4-cyclohexane-dimethanol (CHDM) comonomer), fully aromatic or liquidcrystalline polyesters, biodegradable polyesters, such as thosecomprising butanediol, terephthalic acid and adipic acid residues,poly(cyclohexane-dimethylene terephthalate) homopolymer and copolymers,and homopolymers and copolymers of CHDM and cyclohexane dicarboxylicacid or dimethyl cyclohexanedicarboxylate.

In one embodiment of the present invention, polyester starting materialscomprising at least one alcohol and at least one acid are subjected toesterification in an initial stage of the process. The acid startingmaterial can be a dicarboxylic acid such that the final polyesterproduct comprises at least one dicarboxylic acid residue having in therange of from about 4 to about 15 or from 8 to 12 carbon atoms. Examplesof dicarboxylic acids suitable for use in the present invention caninclude, but are not limited to, terephthalic acid, phthalic acid,isophthalic acid, naphthalene-2,6-dicarboxylic acid,cyclohexanedicarboxylic acid, cyclohexanediacetic acid,diphenyl-4,4′-dicarboxylic acid, dipheny-3,4′-dicarboxylic acid,2,2,-dimethyl-1,3-propandiol, dicarboxylic acid, succinic acid, glutaricacid, adipic acid, azelaic acid, sebacic acid, and mixtures thereof. Inone embodiment, the acid starting material can be a corresponding ester,such as dimethyl terephthalate instead of terephthalic acid.

The alcohol starting material can be a diol such that the finalpolyester product can comprise at least one diol residue, such as, forexample, those originating from cycloaliphatic diols having in the rangeof from about 3 to about 25 carbon atoms or 6 to 20 carbon atoms.Suitable diols can include, but are not limited to, ethylene glycol(EG), diethylene glycol, triethylene glycol, 1,4-cyclohexane-dimethanol,propane-1,3-diol, butane-1,4-diol, pentane-1,5-diol, hexane-1,6-diol,neopentylglycol, 3-methylpentanediol-(2,4), 2-methylpentanediol-(1,4),2,2,4-trimethylpentane-diol-(1,3), 2-ethylhexanediol-(1,3),2,2-diethylpropane-diol-(1,3), hexanediol-(1,3),1,4-di-(hydroxyethoxy)-benzene, 2,2-bis-(4-hydroxycyclohexyl)-propane,2,4-dihydroxy-1,1,3,3-tetramethyl-cyclobutane,2,2,4,4tetramethyl-cyclobutanediol,2,2-bis-(3-hydroxyethoxyphenyl)-propane,2,2-bis-(4-hydroxy-propoxyphenyl)-propane, isosorbide, hydroquinone,BDS-(2,2-(sulfonylbis)4,1-phenyleneoxy))bis(ethanol), and mixturesthereof.

In addition, in one embodiment, the starting materials can comprise oneor more comonomers. Suitable comonomers can include, for example,comonomers comprising terephthalic acid, dimethyl terephthalate,isophthalic acid, dimethyl isophthalate,dimethyl-2,6-naphthalenedicarboxylate, 2,6-naphthalene-dicarboxylicacid, ethylene glycol, diethylene glycol, 1,4-cyclohexane-dimethanol(CHDM), 1,4-butanediol, polytetramethyleneglyocl, trans-DMCD,trimellitic anhydride, dimethyl cyclohexane-1,4 dicarboxylate, dimethyldecalin-2,6 dicarboxylate, decalin dimethanol, decahydronaphthalane2,6-dicarboxylate, 2,6-dihydroxymethyl-decahydronaphthalene,hydroquinone, hydroxybenzoic acid, and mixtures thereof.

In accordance with one embodiment of the present invention, one or moreadditives can be added to the starting materials, the polyester, and/orthe polyester precursors at one or more locations within the process.Suitable additives can include, for example, trifunctional ortetrafunctional comonomers, such as trimellitic anhydride,trimethylolpropane, pyromellitic dianhydride, pentaerythritol, or otherpolyacids or polyols; crosslinking or branching agents; colorant; toner;pigment; carbon black; glass fiber; filler; impact modifier;antioxidant; UV absorbent compound; and oxygen scavenging compound.

In general, the polyester production process according to one embodimentof the present invention can comprise two main stages. The first stagereacts starting materials (also referred to herein as “raw materials” or“reactants”) into monomers and/or oligomers. The second stage furtherreacts the monomers and/or oligomers into the final polyester product.

If the starting materials entering the first stage include acid endgroups, such as, for example, terephthalic acid or isophthalic acid, thefirst stage is referred to as esterification. If the starting materialshave methyl end groups, such as, for example, dimethyl terephthalate ordimethyl isophthalate, the first stage is referred to as ester exchangeor trans-esterification. For simplicity, the term “esterification” asused herein, includes both esterification and ester exchange reactions.According to one embodiment of the present invention, esterification cantake place at a temperature in the range of from about 220° C. to about300° C., or about 235° C. to about 290° C., or 245° C. to 280° C. and apressure of less than about 25 psig, or a pressure in the range of fromabout 1 psig to about 10 psig, or 2 psig to 5 psig. In one embodiment,the average chain length of the monomer and/or oligomer exiting theesterification stage can be less than about 25, from about 1 to about20, or from 5 to 15.

The second stage of the process can be referred to as thepolycondensation stage. The polycondensation stage can be a single stepprocess, or can be divided into a prepolycondensation (orprepolymerization) step and a final (or finishing) polycondensationstep. Generally, longer chain polymers can be produced via a multi-stagepolycondensation process. The polycondensation stage can be carried outat a temperature in the range of from about 220° C. to about 350° C., orabout 240° C. to about 320° C. and a sub-atmospheric (e.g., vacuum)pressure. When polycondensation is carried out in a two-stage process,the prepolymerization (or prepolymer) reactor can convert the monomerexiting the esterification stage into an oligomer having an averagechain length in the range of from about 2 to about 40, from about 5 toabout 35, or from 10 to 30. The finisher reactor then converts theoligomer/polymer mixture into a final polymer product having the desiredaverage chain length.

In accordance with one embodiment of the present invention, theesterification stage can be carried out in an esterification systemcomprising at least one esterification zone and at least onedistillation zone. In the esterification zone, reactants are subjectedto esterification to thereby produce a vapor byproduct and a liquidproduct containing polyester monomers and/or oligomers. A productportion of the liquid product exiting the esterification zone can exitthe esterification system for downstream processing, while arecirculation portion of the liquid product exiting the esterificationzone can be recirculated back to the inlet of the esterification zone.At least a portion of the vapor byproduct exiting the esterificationzone can be routed to the distillation zone, wherein water and alcoholcomponents of the vapor byproduct can be separated. A portion of theseparated alcohol exiting the distillation zone can be combined withsolid acid particles to create a paste, which can then be added to therecirculation portion of the liquid product exiting the esterificationzone. The resulting combined stream can then be reintroduced into theesterification zone, after receiving additional quantities of reactantsand/or additives.

In one embodiment of the present invention, at least a portion of theesterification zone can be defined by equipment that imparts little orno mechanical agitation to the liquid phase of the reaction mediumprocessed therein. Although the liquid phase of the reaction mediumprocessed in the esterification zone may be somewhat agitated by virtueof flowing through the equipment that defines the esterification zone,in one embodiment of the present invention, less than about 50 percent,less than about 25 percent, less than about 10 percent, less than about5 percent, or 0 percent of the agitation of the liquid phase reactionmedium processed in the esterification zone is provided by mechanicalagitation. This is in direct contrast to conventional esterificationprocesses that are carried out in one or more continuous stirred tankreactors (CSTRs) under conditions of extreme mechanical agitation.

As discussed further in detail below, the present invention can employsimple, reliable, and inexpensive equipment for carrying outesterification. For example, in one embodiment of the present invention,at least a portion of the esterification zone can be defined within asimple, reliable, and relatively inexpensive heater, such as, forexample, a shell-and-tube heat exchanger. Further, in anotherembodiment, at least a portion of the esterification zone can be definedwithin a simple, reliable, and relatively inexpensive unagitatedesterification vessel.

Referring now to FIG. 1, an esterification system 10 configured inaccordance with one embodiment of the present invention is illustratedas generally comprising a heat exchanger 12, an esterification vessel14, a distillation column 16, a recirculation loop 18, and a paste tank50. In general, the process carried out in esterification system 10includes the following broad steps: (1) introducing an esterificationfeed into heat exchanger 12; (2) heating and partially esterifying theesterification feed in heat exchanger 12; (3) introducing at least aportion of the heated and partially esterified product from heatexchanger 12 into esterification vessel 14; (4) further esterifying thepartially esterified product from heat exchanger 12 in esterificationvessel 14; (5) separating a liquid product from a vapor byproduct inesterification vessel 14; (6) introducing at least a portion of thevapor byproduct from esterification vessel 14 into distillation column16; (7) separating the vapor byproduct into a predominately wateroverhead stream and a predominately alcohol bottom stream indistillation column 16; (8) combining at least a portion of thepredominantly alcohol bottom stream with solid acid particles to form apaste in paste tank 50; (9) routing a recirculation portion of theliquid product from esterification vessel 14 back to heat exchanger 12via recirculation loop 18; (10) while the recirculation portion of theliquid product is flowing through recirculation loop 18, adding theretofresh alcohol, additive(s), and/or paste from paste tank 50; and (10)withdrawing a product portion of the liquid product from esterificationvessel 14 for further downstream processing.

As indicated above, esterification can be carried out in both heatexchanger 12 and esterification vessel 14 of esterification system 10.Since esterification can be carried out in both heat exchanger 12 andesterification vessel 14, each of these pieces of equipment can bereferred to as “esterification reactors” that each define a portion ofan “esterification zone.” However, because an additional function ofheat exchanger 12 can be to heat the reaction medium processed therein,heat exchanger 12 can also be referred to as a “heater” that defines a“heating zone.” Further, since an additional function of esterificationvessel 14 can be to promote vapor/liquid disengagement, esterificationvessel 14 can also be referred to as a “disengagement vessel” thatdefines a “disengagement zone.” The configuration and operation ofesterification system 10, illustrated in FIG. 1, will now be describedin greater detail.

Referring again to FIG. 1, a recirculated liquid product stream,discussed in more detail below, is transported through a recirculationconduit 100. As illustrated in FIG. 1, the following materials can beadded to the recirculated liquid product stream flowing throughrecirculation conduit 100: (a) fresh alcohol introduced via conduit 104,(b) one or more additives introduced via conduit 106, and (c) anacid-containing paste, discussed in further detail below, introduced viaconduit 108. In another embodiment, at least a portion of the streams inconduits 104 and/or 106 can be added to the stream exitingesterification vessel 14 in conduit 114, which will be discussed indetail below. In yet another embodiment, at least a portion of thestreams in conduits 104 and/or 106 can be introduced directly into ayet-to-be-discussed recirculation pump 40. The fresh alcohol in conduit104 and the acid of the acid-containing paste in conduit 108 can be anyof the alcohols and acids, respectively, discussed above as beingsuitable for use as starting materials in the system of the presentinvention. According to one embodiment, the alcohol can be ethyleneglycol and the acid can be terephthalic acid. The one or more additivesin conduit 106 can be any of the additives discussed above as beingsuitable for used in the system of the present invention.

Generally, the amount of alcohol and acid-containing paste added to therecirculation stream in recirculation conduit 100 can be any amountnecessary to provide the desired production rate and the desiredalcohol-to-acid ratio. In one embodiment of the present invention, themolar alcohol-to-acid ratio of the esterification feed stream exitingrecirculation conduit 100 is in the range of from about 1.005:1 to about10:1, about 1.01:1 to about 8:1, or 1.05:1 to 6:1.

The combined stream exiting recirculation conduit 100 can be introducedas an esterification feed into an inlet 22 of heat exchanger 12 via afeed conduit 110. In heat exchanger 12, the esterification feed/reactionmedium is heated and subjected to esterification conditions. Inaccordance with one embodiment of the present invention, the temperatureincrease of the reaction medium between the inlet 22 and an outlet 24 ofheat exchanger 12 can be at least about 50° F., at least about 75° F.,or at least 85° F. Generally, the temperature of the esterification feedentering inlet 22 of heat exchanger 12 can be in the range of from about220° C. to about 260° C., about 230° C. to about 250° C., or 235° C. to245° C. Generally, the temperature of the esterification product exitingoutlet 24 of heat exchanger 12 can be in the range of from about 240° C.to about 320° C., about 255° C. to about 300° C., or 275° C. to 290° C.The reaction medium in heat exchanger 12 can be maintained at a pressurein the range of from about 5 to about 50 psig, from about 10 to about 35psig, or from 15 to 25 psig.

As discussed previously, heat exchanger 12 can also be considered anesterification reactor because at least a portion of the reaction mediumflowing therethrough can undergo esterification. The amount ofesterification carried out in accordance with the present invention canbe quantified in terms of “conversion.” As used herein, the term“conversion” is used to describe a property of the liquid phase of astream that has been subjected to esterification, wherein the conversionof the esterified stream indicates the percentage of the original acidend groups that have been converted (i.e., esterified) to ester groups.Conversion can be quantified as the number of converted end groups(i.e., alcohol end groups) divided by the total number of end groups(i.e., alcohol plus acid end groups), expressed as a percentage. Whileconversion is used herein, it should be understood that average chainlength, which describes the average number of monomer units that acompound comprises, could also be appropriate for describing thecharacteristics of the streams of the present invention as well.

According to one embodiment, the esterification reaction carried out inheat exchanger 12 can increase the conversion of the reaction mediumbetween inlet 22 and outlet 24 by at least about 5, at least about 10,at least about 15, at least about 20, at least about 30, or at leastabout 50 percentage points. Generally, the esterification feed streamintroduced into inlet 22 of heat exchanger 12 has a conversion of lessthan about 90 percent, less than about 75 percent, less than about 50percent, less than about 25 percent, less than about 10 percent, or lessthan 5 percent, while the esterification product stream exiting outlet24 of heat exchanger 12 has a conversion of at least about 50 percent,at least about 60 percent, at least about 70 percent, at least about 75percent, at least about 80 percent, at least about 85 percent, at leastabout 95 percent, or at least 98 percent.

In one embodiment of the present invention, the esterification reactioncarried out in heat exchanger 12 takes place at a significantly reducedresidence time relative to conventional esterification processes. Forexample, the average residence time of the reaction medium flowingthrough heat exchanger 12 can be less than about 60 minutes, less thanabout 45 minutes, less than about 35 minutes, or less than 20 minutes.This relatively short residence time can even be achieved at high,commercial scale production rates. Thus, in one embodiment, the productstream exits outlet 24 of heat exchanger 12 at a flow rate of at leastabout 10,000 pounds per hour (lb/h), at least about 25,000 lb/h, atleast about 50,000 lb/h, at least about 100,000 lb/h, or at least200,000 lb/h.

Turning now the specific configuration of heat exchanger 12. Inaccordance with one embodiment of the present invention, heat exchanger12 can be a horizontally elongated, shell-and-tube heat exchanger. Aninternal flow passageway through heat exchanger 12 can be defined by theheat exchange tubes through which the reaction medium flows as it isheated and esterified. This internal flow passageway can be consideredto be a “first esterification zone” of esterification system 10.Generally the aggregate volume of the internal flow passageway throughheat exchanger can be in the range of from about 10 to about 1,500 cubicfeet (ft³), about 100 to about 800 ft³, or 200 to 600 ft³. The averageinner diameter of the individual heat exchange tubes can be less thanabout 4 inches, or in the range of from about 0.25 to about 3 inches, or0.5 to 2 inches.

As shown in FIG. 1, a stream of warmed heat transfer medium (HTM) canenter the shell-side of heat exchanger 12 and at least partly surroundat least a portion of the heat exchange tubes in order to heat thereaction medium flowing therethrough. In one embodiment of the presentinvention, the heat transfer coefficient associated with the heating ofthe reaction medium in heat exchanger 12 can be in the range of fromabout 0.5 to about 200 BTU per hour per ° F. per square foot (BTU/h·°F.·ft²), about 5 to about 100 BTU/h·° F.·ft², or from 10 to 50 BTU/h·°F.·ft². The total amount of heat transferred to the reaction medium inheat exchanger 12 can be in the range of from about 100 to about 5,000BTU per pound of reaction medium (BTU/lb), about 400 to about 2,000BTU/lb, or 600 to 1,500 BTU/lb.

As depicted in FIG. 1, the partially esterified product exiting heatexchanger 12 via outlet 24 can be transported to esterification vessel14 via conduit 112. The partially esterified stream in conduit 112 canbe introduced into the internal volume of esterification vessel 14 via afluid inlet 26. As discussed previously, in esterification vessel 14,the partially esterified stream is subjected to further esterificationand phase separation. Thus, the internal volume defined withinesterification vessel can be considered to be a “second esterificationzone” and/or a “disengagement zone.” Generally, the reaction medium inesterification vessel 14 flows substantially horizontally through theinternal volume. As the reaction medium flows away from fluid inlet 26and undergoes esterification, vapor byproducts escape the liquid phaseand flow generally above the liquid phase. The separated liquid productcan exit esterification vessel 14 via a liquid outlet 28, while theseparated vapor byproduct can exit esterification vessel 14 via vaporoutlet 30.

The esterification reaction carried out in esterification vessel 14 canincrease the conversion of the reaction medium processed therein so theliquid product exiting liquid outlet 28 has a conversion that is atleast about 1 percentage point, at least about 2 percentage points, orat least 5 percentage points higher than the conversion of the fluidstream entering fluid inlet 26. Generally, the liquid product exitingliquid outlet 28 of esterification vessel 14 can have conversion of atleast about 80 percent, at least about 85 percent, at least about 90percent, at least 95 percent, or at least about 98 percent.

The conversion achieved in esterification vessel 14 can occur during arelatively short residence time and with little or no heat input. Forexample, the average residence time of the reaction medium inesterification vessel 12 can be less than about 200 minutes, less thanabout 60 minutes, less than about 45 minutes, less than about 30minutes, or less than 15 minutes. Further, the amount of heattransferred to the reaction medium in esterification vessel 14 can beless than about 100 BTU per pound of reaction medium (BTU/lb), less thanabout 20 BTU/lb, less than about 5 BTU/lb, or less than 1 BTU/lb.

With minimal or no heat input in esterification vessel 14, the averagetemperature of the liquid product exiting liquid outlet 28 ofesterification vessel 14 can be within about 50° C., about 30° C., about20° C., or 15° C. of the average temperature of the fluid enteringesterification vessel 14 via fluid inlet 26. Generally, the averagetemperature of the liquid product exiting liquid outlet 28 ofesterification vessel 14 can be in the range of from about 220° C. toabout 320° C., about 240° C. to about 300° C., or about 250° C. to about275° C.

Turning now to the specific configuration of esterification vessel 14.In the embodiment illustrated in FIG. 1, esterification vessel 14 is asubstantially empty, unagitated, unheated, generally cylindrical,horizontally elongated vessel. Esterification vessel 14 and can have alength-to-diameter (L:D) ratio of less than about 10:1, in the range offrom about 1.25:1 to about 8:1, about 1.5:1 to about 6:1, or 2:1 to4.5:1. In one embodiment, fluid inlet 26, liquid outlet 28, and vaporoutlet 30 are spaced from on another in a manner that providessufficient esterification and enhances disengagement/separation of thevapor, liquid, and foam phases. For example, liquid outlet 28 and vaporoutlet 30 can be horizontally spaced from the fluid inlet 26 by at leastabout 1.25 D, at least about 1.5 D, or at least 2.0 D. Further, liquidoutlet 28 and vapor outlet 30 can be vertically spaced from one anotherby at least about 0.5 D, at least about 0.75 D, or at least 0.95 D.

As illustrated in FIG. 1, esterification vessel 14 can comprise a fluiddistributor 32 to aid in the effective distribution of the feed toesterification vessel 14. In the embodiment illustrated in FIG. 1, fluiddistributor is simply a substantially horizontally extending pipe havinga downwardly curved distal end that defines fluid inlet 26 with adownwardly facing orientation. Alternatively, fluid distributor 32 candefine a plurality of openings for discharging the partially esterifiedfeed at multiple horizontally spaced locations in esterification vessel14. In one embodiment of the present invention, the average depth of thereaction medium in esterification vessel 14 is maintained at less thanabout 0.75 D, less than about 0.50 D, less than about 0.25 D, or lessthan 0.15 D as it travels substantially horizontally throughesterification vessel 14.

As shown in FIG. 1, upon entering esterification vessel 14, the reactionmedium exiting fluid distributor 32 can begin to foam as the vaporbubbles disengage from the liquid portion of the reaction medium.Generally, foam production can decrease along the length ofesterification vessel 14 as the vapor disengages from the liquid phaseof the reaction medium so that, in one embodiment, substantially no foamexits liquid outlet 28 and/or vapor outlet 30 of esterification vessel14.

To help ensure that substantially no foams exits vapor outlet 30 ofesterification vessel 14, a downwardly extending baffle 34 can beemployed in esterification vessel 14. Baffle 34 can generally bedisposed between fluid inlet 26 and vapor outlet 30 of esterificationvessel 14, but closer to vapor outlet 30 than to fluid inlet 26. Baffle34 can extend downwardly from the top of esterification vessel 14proximate vapor outlet 30 and can function to physically block the flowof foam, if any, towards vapor outlet 30. In one embodiment of thepresent invention, baffle 34 can present a bottom edge vertically spacedat least about 0.25 D, at least about 0.5 D, or at least 0.75 D from thebottom of esterification vessel 14. In the embodiment illustrated inFIG. 1, baffle includes a downwardly extending portion 36 and alaterally extending portion 38. Downwardly extending portion 36 canextend downwardly from a location proximate vapor outlet 30, whilelaterally extending portion 38 can extend transversely from the bottomend of downwardly extending portion 36 to a location generally undervapor outlet 30.

The total internal volume defined within esterification vessel 14 candepend on a number of factors, including, for example, the overallhydrodynamic requirements of esterification system 10. In one embodimentof the present invention, the total internal volume of esterificationvessel 14 can be at least about 25 percent, at least about 50 percent,at least about 75 percent, at least about 100 percent, or at least 150percent of the total internal volume of recirculation loop 18, describedin further detail below. In yet another embodiment of the presentinvention, the total internal volume of esterification vessel 14 can beat least about 25 percent, at least about 50 percent, at least about 75percent, or at least 150 percent of the aggregate internal volume ofrecirculation loop 18, the flow passageway within heat exchanger 12, andproduct conduit 112.

Referring again to FIG. 1, a liquid ester product can exit liquid outlet28 of esterification vessel 14 and can thereafter be introduced intorecirculation loop 18. Recirculation loop 18 defines a flow passagewayfrom liquid outlet 28 of esterification vessel 14 to inlet 22 of heatexchanger 12. Recirculation loop 18 generally comprises a liquid productconduit 114, a recirculation pump 40, a pump discharge conduit 116,recirculation conduit 100, pressure reducer 20, and feed conduit 110.The liquid ester product discharged from esterification vessel 14 canflow initially through product conduit 114 to the suction ofrecirculation pump 40. The stream exiting pump 40 can be passed thoughpump discharge conduit 116 and thereafter split into a product portiontransported via ester product conduit 118 and a recirculation portiontransported via recirculation conduit 100. The splitting of the streamexiting pump 40 can be carried out so that the ratio of the mass flowrate of the recirculation portion in conduit 100 to the mass flow rateof the product portion in conduit 118 can be in the range of from about0.25:1 to about 30:1, about 0.5:1 to about 20:1, or 2:1 to 15:1. Aspreviously discussed, the recirculation portion in conduit 100 caneventually be employed as the feed to heat exchanger 12, after theaddition of fresh alcohol via conduit 104, additive(s) via conduit 106,and/or acid-containing paste via conduit 108.

The product portion of the liquid ester product in conduit 118 can berouted to a downstream location for further processing, storage, orother use. In one embodiment, at least a fraction of the product portionin conduit 118 can be subjected to further esterification in a secondesterification zone. In another embodiment, at least part of the productportion in conduit 118 can be subjected to polycondensation in adownstream polycondensation zone.

As illustrated in FIG. 1, the vapor stream exiting vapor outlet 30 ofesterification vessel 14 via conduit 120 can be routed to a fluid inlet42 of distillation column 16. The vapor byproduct stream in conduit 120can comprise water and alcohol. The water and alcohol can besubstantially separated from one another in distillation column 16 tothereby produce a predominately water overhead vapor stream exitingdistillation column 16 via overhead outlet 44 and a predominatelyalcohol bottom liquid stream exiting distillation column 16 via loweroutlet 46. Distillation column 16 can be any device capable ofseparating a stream into a predominantly vapor overhead product and apredominantly liquid bottoms product based on the relative volatilitiesof the components of the feed stream. Distillation column 16 cancomprise internals such as, for example, trays, random packing,structured packing, or any combination thereof.

According to one embodiment of the present invention, the predominantlywater overhead vapor stream exiting distillation column 16 via overheadoutlet 44 can comprises at least about 50 mole percent, at least about60 mole percent, or at least 75 mole percent water. The overhead vaporproduct discharged from outlet 44 of distillation column 16 can berouted via conduit 122 to subsequent processing, storage, or disposal,such as, for example, a wastewater processing unit or a disposal meansemploying, for example, incineration.

The predominately alcohol bottom liquid stream exiting distillationcolumn 14 via lower outlet 46 can comprise at least about 50 molepercent, at least about 60 mole percent, or at least 75 mole percentalcohol (e.g., ethylene glycol). In one embodiment of the presentinvention, the predominantly alcohol stream withdrawn from lower outlet46 of distillation column 16 can have a temperature of at least about150° C., in the range of from about 175° C. to about 250° C., or 190° C.to 230° C. and a pressure in the range of from about 0.25 psig to about50 psig, about 0.5 psig to about 35 psig, or 1 psig to 25 psig. As shownin FIG. 1, the liquid stream discharged from lower outlet 46 ofdistillation column can be transported in conduit 124 and thereaftersplit into a first alcohol-containing portion carried in conduit 126 anda second alcohol-containing portion carried in conduit 102. Theseparated liquid stream from conduit 124 can be split in a manner suchthat the mass flow rate of the second portion of the alcohol-containingstream in conduit 102 can be at least about 25 percent, at least about50 percent, or at least 75 percent of the mass flow rate of the liquidproduct in conduit 124. The first alcohol-containing portion in conduit126 can then be routed downstream for further processing, storage,and/or reuse.

In one embodiment of the present invention, at least a portion of thesecond alcohol-containing stream in conduit 102 can be mixed with solidacid particles in order to form the previously-discussed acid-containingpaste. The solid acid particles can be any of the acids discussed aboveas being suitable for use as starting materials in the system of thepresent invention. To optimize the final properties (e.g., viscosity) ofthe acid-containing paste, the alcohol-containing stream in conduit 102can be cooled via indirect heat exchange means 48, as shown in FIG. 2.The indirect heat exchange means 48 can be any heat exchange means knownin the art capable of reducing the temperature of the alcohol-containingstream in conduit 102 in the range of from about 25° C. to about 150°C., or about 50° C. to about 100° C. from the temperature of the streamwhen it exits distillation column 16. In general, the temperature of thecooled alcohol-containing stream in conduit 128 can be in the range offrom 80° C. to 170° C., about 90° C. to about 150° C., or 100° C. to125° C.

Referring back to FIG. 1, the cooled alcohol-containing stream inconduit 128 enters a fluid inlet 52 of paste tank 50. Solid acidparticles (i.e., particles of terephthalic acid) in conduit 130 can beintroduced into paste tank 50 via upper acid inlet 54. A mechanicalagitator 58 thoroughly mixes the solid acid particles and thealcohol-containing stream in order to produce a paste comprising in therange of from about 30 to about 99 weight percent solids, about 50 toabout 90 weight percent solids, or 55 to 85 weight percent solids. Hotpaste having a temperature in the range of from about 80° C. to about170° C., about 90° C. to about 150° C., or 100° C. to 125° C. exitslower paste outlet 56 of paste tank 50 and enters conduit 108. In oneembodiment, esterification system 10 can employ a temperature controlsystem 60 to ensure the hot paste in conduit 108 is maintained at theproper temperature. Temperature control system 60 can be operable tomodify the temperature of the hot paste in conduit 108 by adjustingamount of cooling provided by indirect heat exchange means 48 to thealcohol-containing stream in conduit 128. For example, if thetemperature of the hot paste in conduit 108 is higher than desired,temperature indicator 62 sends a signal 64 to indirect heat exchangemeans 48, which subsequently increases the amount of cooling provided tothe alcohol containing stream in conduit 102 to thereby ultimatelydecrease the temperature of the hot paste in conduit 108.

As illustrated in FIG. 1, the hot paste in conduit 108 can be added torecirculation loop 18 downstream of the additional alcohol and additiveinjection points. Further, it can be advantageous to introduce the pasteinto the top portion of recirculation conduit 100 in order to expeditethe dissolution of the solid acid reactant as it descends into therecirculation stream. The presence of polyester monomers and/oroligomers in the recirculation stream can also enhance the dissolutionof the solid acid particles added to recirculation conduit 100. In oneembodiment of the present invention, the stream in recirculation conduit100 can have an average chain length in the range of from about 1 toabout 20, about 2 to about 18, or 5 to 15.

Conventional esterification systems require substantial cooling of theacid-containing paste prior to introduction into the recirculated esterproduct stream. However, in accordance with one embodiment of thepresent invention, when combined with the recirculated esterificationproduct stream flowing through conduit 116 in recirculation conduit 100,the temperature of the acid containing paste is at least about 25° C.,at least about 40° C., or at least 50° C. cooler, but not more thanabout 100° C., not more than about 75° C., not more than 60° C., coolerthan the temperature of the alcohol stream when it was withdrawn fromlower outlet 46 of distillation column 16. In one embodiment, thetemperature of the acid-containing paste when combined with therecirculated ester product stream in recirculation conduit 100 is in therange of from about 80° C. to about 170° C., about 90° C. to about 150°C., or 100° C. to 135° C.

Numerical Ranges

The present description uses numerical ranges to quantify certainparameters relating to the invention. It should be understood that whennumerical ranges are provided, such ranges are to be construed asproviding literal support for claim limitations that only recite thelower value of the range as well as claims limitation that only recitethe upper value of the range. For example, a disclosed numerical rangeof 10 to 100 provides literal support for a claim reciting “greater than10” (with no upper bounds) and a claim reciting “less than 100” (with nolower bounds).

DEFINITIONS

As used herein, the terms “a,” “an,” “the,” and “said” means one ormore.

As used herein, the term “agitation,” refers to work dissipated into areaction medium causing fluid flow and/or mixing.

As used herein, the term “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itself,or any combination of two or more of the listed items can be employed.For example, if a composition is described as containing components A,B, and/or C, the composition can contain A alone; B alone; C alone; Aand B in combination; A and C in combination; B and C in combination; orA, B, and C in combination.

As used herein, the terms “comprising,” “comprises,” and “comprise” areopen-ended transition terms used to transition from a subject recitedbefore the term to one or elements recited after the term, where theelement or elements listed after the transition term are not necessarilythe only elements that make up of the subject.

As used herein, the terms “containing,” “contains,” and “contain” havethe same open-ended meaning as “comprising,” “comprises,” and“comprise,” provided below.

As used herein, the term “distillative separation” refers to separatingone or more chemical substances from one or more other chemicalsubstances based on the relative volatilities of the substances beingseparated.

As used herein, the terms “having,” “has,” and “have” have the sameopen-ended meaning as “comprising,” “comprises,” and “comprise,”provided above

As used herein, the terms “including,” “includes,” and “include” havethe same open-ended meaning as “comprising,” “comprises,” and“comprise,” provided above.

As used herein, the term, “mechanical agitation,” refers to agitation ofa reaction medium caused by physical movement of a rigid or flexibleelement(s) against or within the reaction medium.

As used herein, the term “reaction medium” refers to any mediumsubjected to chemical reaction.

As used herein, the term “residue” refers to the moiety that is theresulting product of the chemical species in a particular reactionscheme or subsequent formulation or chemical product, regardless ofwhether the moiety is actually obtained from the chemical species.

CLAIMS NOT LIMITED TO DISCLOSED EMBODIMENTS

The preferred forms of the invention described above are to be used asillustration only, and should not be used in a limiting sense tointerpret the scope of the present invention. Modifications to theexemplary embodiments, set forth above, could be readily made by thoseskilled in the art without departing from the spirit of the presentinvention.

The inventors hereby state their intent to rely on the Doctrine ofEquivalents to determine and assess the reasonably fair scope of thepresent invention as pertains to any apparatus not materially departingfrom but outside the literal scope of the invention as set forth in thefollowing claims.

What is claimed is:
 1. A process comprising: (a) producing analcohol-containing liquid from a distillation zone; (b) mixing at leasta portion of said alcohol-containing liquid with solid particles of anacid to thereby provide a paste; (c) combining said paste with arecirculation liquid to thereby provide an esterification feed; and (d)subjecting at least a portion of said esterification feed toesterification in an esterification zone, wherein the temperature ofsaid paste when combined with said recirculation liquid is not more than150° C. cooler than the temperature of said alcohol-containing liquidwhen exiting said distillation zone, wherein said alcohol-containingliquid exits the distillation zone at a temperature in the range of fromabout 175° C. to about 250° C., wherein the temperature of the saidpaste when combined with said recirculation liquid is in the range of80° C. to about 170° C.
 2. The process of claim 1, wherein thetemperature of said paste when combined with said recirculation liquidis at least 25° C. cooler than the temperature of saidalcohol-containing liquid when exiting said distillation zone.
 3. Theprocess of claim 1, wherein the temperature of said paste when combinedwith said recirculation liquid is not more than 100° C. cooler than thetemperature of said alcohol-containing liquid when exiting saiddistillation zone.
 4. The process of claim 1, wherein saidalcohol-containing liquid exits said distillation zone at a temperaturein the range of from 190° C. to 230° C., wherein the temperature of saidpaste when combined with said recirculation liquid is in the range of90° C. to about 150° C., wherein said esterification is carried out at atemperature in the range of from 220° C. to 300° C.
 5. The process ofclaim 1, further comprising cooling said alcohol-containing liquid priorto said mixing of step (b).
 6. The process of claim 1, wherein saidcooling includes reducing the temperature of said alcohol-containingliquid by about 25° C. to about 150° C.
 7. The process of claim 5,wherein said cooling includes reducing the temperature of saidalcohol-containing liquid by about 50° C. to about 100° C. to atemperature in the range of from 90 to 150° C.
 8. The process of claim1, wherein said paste comprises in the range of from about 50 to about95 weight percent of said solids.
 9. The process of claim 1, whereinsaid recirculation liquid aids in the dissolution of said paste.
 10. Theprocess of claim 1, wherein said recirculation liquid has an averagechain length in the range of from about 2 to about
 18. 11. The processof claim 1, wherein said alcohol is a diol and said acid is a diacid.12. The process of claim 1, wherein said alcohol is ethylene glycol andsaid acid is terephthalic acid.
 13. The process of claim 1, wherein anesterification product having a conversion of at least about 50 percentis produced from said esterification zone.
 14. The process of claim 13,wherein said recirculation liquid comprises at least a portion of saidesterification product.
 15. The process of claim 1, wherein at least aportion of said esterification zone is defined within a heat exchanger.16. The process of claim 15, wherein said heat exchanger comprises tubesat least partly surrounded by a heat transfer medium, whereinesterification zone is at least partly defined within said tubes. 17.The process of claim 1, further comprising, optionally, agitating saidreaction medium in said esterification zone, wherein less than about 50percent of the agitation is provided by mechanical agitation.
 18. Theprocess of claim 1, further comprising producing an esterificationproduct from said esterification zone and subjecting at least a portionof said esterification product to vapor/liquid separation in adisengagement zone to thereby produce a first liquid product and a firstvapor product.
 19. The process of claim 18, wherein said recirculationliquid comprises at least a portion of said first liquid product. 20.The process of claim 18, wherein further esterification is carried outin said disengagement zone so that the conversion of said first liquidproduct is at least 1 percentage point higher than the conversion ofsaid esterification product.
 21. The process of claim 20, wherein saidfirst liquid product has an conversion of at least about 70 percent. 22.The process of claim 18, wherein said disengagement zone is definedwithin a horizontally elongated disengagement vessel.
 23. The process ofclaim 18, further comprising, optionally, agitating a liquid phase insaid disengagement zone, wherein less than about 50 percent of theagitation is provided by mechanical agitation.
 24. The process of claim18, further comprising separating said first liquid product into aproduct portion and a recirculated portion, wherein said recirculationliquid comprises at least a portion of said recirculated portion. 25.The process of claim 24, further comprising subjecting said productportion to further esterification in a second esterification zone and/orto polycondensation in a polycondensation zone.
 26. The process of claim18, further comprising introducing at least a portion of said firstvapor into said distillation zone and separating said first vaporportion into a second vapor stream and said alcohol-containing liquid.27. The process of claim 25, wherein said second vapor stream comprisesat least about 50 mole percent water and said alcohol-containing liquidcomprises at least about 50 mole percent ethylene glycol.
 28. A processcomprising: (a) subjecting an esterification feed comprisingterephthalic acid and ethylene glycol to esterification in a firstesterification zone defined within the tubes of a shell-and-tube heatexchanger to thereby produce a first product having a conversion of atleast about 50 percent; (b) subjecting at least a portion of said firstproduct to vapor/liquid separation in a disengagement zone definedwithin a disengagement vessel to thereby produce a second liquid productand a second vapor product, wherein said second liquid product has ahigher conversion than said first product; (c) subjecting at least aportion of said second vapor product to distillative separation in adistillation zone to thereby produce a third vapor product and a thirdliquid product, wherein said third vapor product comprises at leastabout 50 mole percent water and said third liquid product comprises atleast about 50 mole percent ethylene glycol; (d) mixing at least aportion of said third liquid product with solid particles ofterephthalic acid to thereby provide a paste; (e) combining at least aportion of said paste with a recirculation liquid to thereby provide acombined stream, wherein the temperature of said paste when combinedwith said recirculation liquid is not more than 150° C. cooler than thetemperature of said third liquid when exiting said distillation zone,wherein said recirculation liquid comprises at least a portion of saidsecond liquid product, wherein said esterification feed comprises atleast a portion of said combined stream, wherein said third liquid exitsthe distillation zone at a temperature in the range of from about 175°C. to about 250° C., wherein the temperature of the said paste whencombined with said recirculation liquid is in the range of 80° C. toabout 170° C.
 29. The process of claim 28, further comprising cooling atleast a portion of said third liquid prior to said mixing of step (d).30. The process of claim 29, wherein said cooling includes reducing thetemperature of said third liquid by at least about 25° C.
 31. Theprocess of claim 29, wherein said cooling includes reducing thetemperature of said third liquid by an amount in the range of from about50° C. to about 100° C.
 32. The process of claim 28, wherein thetemperature of said paste when combined with said recirculation liquidis at least about 25° C. cooler than the temperature of said thirdliquid when exiting said distillation zone.
 33. The process of claim 32,wherein the temperature of said paste when combined with saidrecirculation liquid is not more than 100° C. cooler than thetemperature of said third liquid when exiting said distillation zone.34. The process of claim 28, wherein said third liquid exits saiddistillation zone at a temperature in the range of from 190° C. to 230°C., wherein the temperature of said paste when combined with saidrecirculation liquid is in the range of 90° C. to about 150° C., whereinsaid esterification is carried out at a temperature in the range of from220° C. to 300° C.
 35. The process of claim 28, wherein saidrecirculation liquid aids in the dissolution of said solid particles ofterephthalic acid.
 36. The process of claim 28, wherein saiddisengagement vessel defines a fluid inlet for receiving said firstproduct, a vapor outlet for discharging said second vapor product, and aliquid outlet for discharging said second liquid, wherein said vapor andliquid outlets are horizontally spaced from said fluid inlet by adistance greater than the maximum vertical dimension of saiddisengagement zone.
 37. The process of claim 36, wherein saiddisengagement vessel has a length-to-diameter ratio in the range of fromabout 1.25:1 to about 8:1.
 38. The process of claim 28, furthercomprising, optionally, agitating an esterification reaction medium insaid esterification zone and/or agitating a disengagement medium in saiddisengagement zone, wherein less than about 50 percent of the totalagitation in said esterification and disengagement zones is provided bymechanical agitation.